The present invention relates generally to focusing control and indication of information relating to an object and so on, and may be applicable particularly to vide cameras.
Known is a video camera having a zooming apparatus for allowing selection between an automatic focus mode and a manual focus mode, which zooming apparatus comprises a front lens system, a rear lens system, and a zoom lens system provided therebetween. Various types of zooming apparatus have been developed theretofore. One previous arrangement is that in the manual focus mode the front lens system is moved in the directions of the axis of the zooming apparatus by manual rotation of a focus ring after the rear lens system is moved to and fixed at a predetermined position and in the automatic focus mode the rear lens system is automatically moved in the same directions after the front lens system is moved up to a predetermined position. On the plus side, defocusing does not occur during zooming because the rear lens system is kept at the predetermined position and on the other hand this requires that two focusing mechanisms are provided for the front and the rear lens systems, respectively, resulting in a complex arrangement.
Another known approach is that in the automatic focus mode and the manual focus mode only the rear lens system is moved for focus adjustment so that its arrangement becomes simple. However, this approach provides a disadvantage that difficulty is encountered to keep the object in focus in the case that the zooming operation is performed after focused to the object which is located at a finite distance from the video camera. The reason of the off-focusing will be described with reference to FIG. 5 which is a graphic diagram showing the relationship between the focal length and the rear lens system position on zooming in the focusing state. If taking a picture of an object at 2 meter's distance under the condition that a focal length is fa, the focusing state can be obtained by positioning the rear lens system to xa (point A). When the zoom-down is then performed so that the focal length becomes fc, the rear lens system position is not shifted to xb (point B) and remains at xa (point C), resulting in the off-focusing.
In addition, such a zooming apparatus in which the focusing operation is effected by moving only the rear lens system has a disadvantage in that it is difficult to accurately indicate the distance to the object because the distance to the focused object is determined by the position of the zoom lens unit besides the rear lens system position. This disadvantage causes a poor operativity.
Furthermore, an important problem in optical systems relates to the need for focusing control when a conversion lens is mounted thereto. Generally, the focal length is changed in response to the mounting of a conversion lens such as telephoto conversion lens and wide-angle conversion lens, and therefore this requires to a further adjustment to again bring the object into focus and is troublesome. Thus, a further improvement would be required from the view point of elimination of the additional trouble and accurate image pickup.